Nano-phase metals and ceramics derived from nanometer-scaled particles are known to exhibit unique physical and mechanical properties. The novel properties of nano-crystalline materials are the result of their small residual pore sizes (small intrinsic defect sizes), limited grain sizes, phase or domain dimensions, and large fraction of atoms residing in interfaces. In a multi-phase material, limited phase dimensions could imply a limited crack propagation path if the brittle phase is surrounded by ductile phases so the cracks in a brittle phase would not easily reach a critical crack size. In addition, dislocation movement distances in a metal could be limited in ultra fine metallic domains. Even with only one constituent phase, nano-crystalline materials may be considered as two-phase materials, composed of distinct interfaces and crystalline phases. The possibilities for reacting, coating, and mixing various types of nano materials create the potential for fabricating new composites with nanometer-sized phases and novel properties.
The interest in ultra-fine particles or clusters (d&lt;100 nm) is due to the unique processing characteristics as well as performance properties exhibited by small particles of metals, semiconductors and ceramics. Ultra-fine particles with a narrow size distribution have enormous potential in ceramic processing. For example, a green density of 75% has been achieved by compaction of nano-crystalline titania prepared by inert gas condensation of metal vapors.
Mono-dispersed particles are known to form a more uniform green micro-structure, which allows for a better control of the micro-structure during densification. In addition, smaller particles can be sintered at much lower temperatures. Not only the structure, but also the mechanical, electronic, optical, magnetic and thermal properties of nano-crystalline materials are different from those exhibited by their bulk counterparts. Specifically, ceramics fabricated from ultra-fine particles are known to possess high strength and toughness because of the ultra-small intrinsic defect sizes and the ability for grain boundaries to undergo a large plastic deformation. Additionally, ultra-fine grained metals could exhibit unusually high strength and hardness.
For a review on nano-phase materials please refer to A. N. Goldstein, "Handbook of Nanophase Materials," Marcel Dekker, Inc., New York, 1997. The techniques for the generation of nanometer-sized particles may be divided into three broad categories: vacuum, gas-phase, and condensed-phase synthesis. Vacuum synthesis techniques include sputtering, laser ablation, and liquid-metal ion sources. Gas-phase synthesis includes inert gas condensation, oven sources (for direct evaporation into a gas to produce an aerosol or smoke of clusters), laser-induced vaporization, laser pyrolysis, and flame hydrolysis. Condensed-phase synthesis includes reduction of metal ions in an acidic aqueous solution, liquid phase precipitation of semiconductor clusters, and decomposition-precipitation of ionic materials for ceramic clusters. Other methods include mix-alloy processing, chemical vapor deposition (CVD), and sol-gel techniques.
All of these techniques have one or more of the following problems or shortcomings:
(1) Most of these prior-art techniques suffer from a severe drawback: extremely low production rates. It is not unusual to find a production rate of several grams a day in a laboratory scale device. Vacuum sputtering, for instance, only produces small amounts of particles at a time. Laser ablation and laser-assisted chemical vapor deposition techniques are also well-known to be excessively slow processes. These low production rates, resulting in high product costs, have severely limited the utility value of nano-phase materials. There is, therefore, a clear need for a faster, more cost-effective method for preparing nanometer-sized powder materials. PA1 (2) Condensed-phase synthesis such as direct reaction of metallic silicon with nitrogen to produce silicon nitride powder requires pre-production of metallic silicon of high purity in finely powdered form. This reaction tends to produce a silicon nitride powder product which is constituted of a broad particle size distribution. Furthermore, this particular reaction does not yield a product powder finer than 100 nm (nanometers) except with great difficulty. Due to the limited availability of pure metallic silicon in finely powdered form, the use of an impure metallic powder necessarily leads to an impure ceramic product. These shortcomings are true of essentially all metallic elements, not just silicon. PA1 (3) Some processes require expensive precursor materials to ceramic powders and could result in harmful gas that has to be properly disposed of. For instance, the reaction scheme of 3SiCl.sub.4 +4NH.sub.3 =Si.sub.3 N.sub.4 +12HCl involves the utilization of expensive SiCl.sub.4 and produces dangerous HCl gas. PA1 (4) Most of the prior-art processes are capable of producing a particular type of metallic or ceramic powder at a time, but do not permit the preparation of a uniform mixture of two or more types of nano-scaled powders at a predetermined proportion. PA1 (5) Most of the prior-art processes require heavy and/or expensive equipment (e.g., a high power laser source or a plasma generator), resulting in high production costs. In the precipitation of ultra fine particles from the vapor phase, when using thermal plasmas or laser beams as energy sources, the particle sizes and size distribution cannot be precisely controlled. Also, the reaction conditions usually lead to a broad particle size distribution as well as the appearance of individual particles having diameters that are multiples of the average particle size. PA1 1. A wide variety of nano-scaled particles can be readily produced. The starting metal materials can be selected from any element in the periodic table that is considered to be metallic. The ceramic materials can be selected from the group of hydride, oxide, carbide, nitride, chloride, boride, silicide and sulfite and combinations thereof. No known prior-art technique is so versatile in terms of readily producing so many different types of nano-scaled ceramic, metallic and inter-metallic compound powders. PA1 2. The starting materials can be a mixture of alloys, pure elements, ceramics, and/or intermetallic compounds. When broken up into nano-sized clusters, these constituents will become uniformly dispersed and some of them are capable of reacting with reactant gas species intentionally added to induce predetermined chemical reactions. PA1 3. The presently invented rolling-type high-energy planetary ball mill, as explained at a later section, exhibits much improved crushing forces and frequencies of the grinding balls. This feature makes the process fast and effective and now makes it possible to mass produce nano-sized ceramic powders cost-effectively. PA1 4. The apparatus needed to carry out the invented process is simple and easy to operate. It does not require the utilization of expensive equipment. The over-all product costs are very low.
The method of ball milling has a great potential to become free from most of the above cited deficiencies. However, the conventional ball milling (mechanical attrition and grading) processes have the disadvantages that powders can only be produced up to a certain fineness (down to 0.5 .mu.m) and with a relatively broad particle-size distribution. A laboratory-scale conventional ball mill is capable of producing only several kilograms of nano-scaled powders in approximately 100 hours. Fortunately, a scaled-up ball mill is known to have the capability to produce fine powders in "tonnage" quantity. If the power and efficiency of a ball mill can be significantly improved, ball milling can become a mass production method for the preparation of nano-scaled powders. In the past decade, it has been well demonstrated in research laboratories that the grinding effect of a high-energy planetary ball mill is sufficient for atomic scale combinations and chemical reactions between materials to be readily achieved. Amorphous phases, intermetallic compounds and solid solutions with a wide range of solubilities can be formed by ball milling. Pure elements, compounds and ceramics can be ground into manometer particles. For a review on this topic, please refer to: C. C. Koch, "The Synthesis and Structure of Nanocrystalline Materials Produced by Mechanical Attrition: A Review," NanoStructured Materials, 2 (1993) pp. 109-129.
Prior art grinding mills are disclosed in the following U.S. Pat. Nos. 459,662 (Sep. 1, 1891 to J. H. Pendleton); 569,828 (Oct. 20, 1896 to A. Herzfeld); 1,144,272 (Jun. 22, 1915 to A. L. West); 1,951,823 (Apr. 7, 1930 to W. P. Eppers); 2,209,344 (Jul. 30, 1940 to N. L. Mathews); 2,387,095 (Oct. 16, 1945 to E. M. Shideler and H. L. Bloxon); 2,874,911 (Feb. 24, 1959 to F. Limb); 2,937,814 (May 24, 1960 to A. Joisel); 3,190,568 (Jun. 22, 1965 to D. Freedman); 3,513,604 (May 26, 1970 to M. Matsunaga and H. Kobayashi); 3,529,780 (Sep. 22, 1970 to C. H. Wilkinson); 3,876,160 (Apr. 8, 1975 to R. Bloch); 3,981,488 (Sep. 21, 1976 to S. Ratowsky); 4,057,191 (Nov. 8, 1977 to I. Ohno); 4,601,431 (Jul. 22, 1986 to Y. Watanabe, et al.); 4,676,439 (Jun. 30, 1987 to K. Saito, et al.); 4,679,737 (Jul. 14, 1987 to T. R. Romer); 4,679,736 (Jul. 14, 1987 to J. Orlando); 4,720,050 (Jan. 19, 1988 to H. A. Eberhardt); 4,844,355 (Jul. 4, 1989 to P. B. Kemp, Jr.); 4,887,773 (Dec. 19, 1989 to J. C. Mehltretter). The drawbacks or shortcomings of these and other prior art grinding mills and related methods have been reviewed and summarized by R. L. Gamblin in U.S. Pat. Nos. 5,029,760 (Jul. 9, 1991), 5,205,499 (Apr. 27, 1993), 5,356,084 (Oct. 18, 1994) and 5,375,783 (Dec. 27, 1994). More recent work on high-energy grinding mills can be found in the following U.S. Patents and related foreign patents cited therein: U.S. Pat. Nos. 5,232,169 (Aug. 3, 1993 to K. Kaneko, et al.); 5,522,558 (Jun. 4, 1996 to K. Kaneko); and 5,386,615 (Jan. 24, 1995 to A. Calka and B. W. Ninham).
All these prior art grinding ball mills have one or more shortcomings in terms of power, efficiency, capacity, production rate, bulkiness, and/or equipment costs. Some of these grinding mills are not suitable for use in producing nanometer-scaled powder particles. Among the prior-art mills, the high-energy ball mills that involve planetary motions of mill pots appear to have the greatest potential for use in the preparation of nano-sized particles and novel alloys that would otherwise be difficult to fabricate. As discussed in a later section (DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS), if the actions of impacting, squeezing and rubbing in a high-energy ball mill can be further improved, ball milling can become an ideal process for the mass production of nanometer-scaled powder materials.
Accordingly, one object of the present invention is to provide an improved ball milling apparatus and method for producing ultra fine powder materials.
Another object of the present invention is to provide an apparatus and method for producing ultra fine powder materials at a high production rate.
A specific object of the present invention is to provide an apparatus and method for producing nanometer-sized particles.
Still another object of the present invention is to provide a low-cost apparatus for producing a wide range of ultra fine powder materials at a high production rate.
A further object of the present invention is to provide an apparatus for producing a mixture of ultra fine powder materials which are well mixed and well dispersed at a predetermined proportion.